1. Field of the Invention
The present invention relates to a pulse width modulation circuit which modulates a pulse width according to input data.
2. Description of Related Art
Laser beam printers (LBPs) and digital copiers having a laser have been widely used because of their high-resolution, silent, and high-speed features. Pulse width modulation (PWM) is a main technique for controlling print tone in these devices. The pulse width modulation controls the tone or density of each printing dot by pulse width.
There are broadly two systems for conventional pulse width modulation: counter and delay. A counter system is disclosed in Japanese Unexamined Patent Publication No. 56-69929 (Imai et al.), for example. In the counter system, a counter counts reference clocks, and a comparator compares the count value with a digital pulse data value or input data. When the both values match, the system changes a PWM output.
On the other hand, a delay system is disclosed in Japanese Unexamined Patent Publication No. 06-177723 (Oshima), for example. The delay system inputs a reference clock to a delay circuit, selects the one corresponding to a digital pulse data value from a plurality of delay signals output from the delay circuit, and changes a PWM output.
However, the counter system and the delay system used for conventional pulse width modulation circuits have a problem that high-speed, high-accuracy operation is difficult.
For example, a case of generating a PWM pulse with 100 MHz, 8-bit resolution is as follows. In this case, a resolution is, when converted to time, 1 s/100 MHz/28≈40 ps. This indicates that it is necessary in the counter system that a counter circuit operates with a clock cycle tCK=40 ps or less, and in the delay system that a delay time in each cell stage constituting a delay circuit is 40 ps or less. Current CMOS technology can hardly implement such performance.
The limitation of 40 ps may be relaxed by performing interleaving process. However, this increases the number of divisions in interleave, which generates new problems such as an increase in the circuit size and a relative variation in each stage.